The present invention pertains to production well completion and more particularly to connecting well casings and their alignment to prevent thread and seal damage.
In oil well completion, or any type of well, such as water, gas, etc., it is standard practice to sink a casing once the wellbore has been drilled. A casing is designed to preserve the integrity of the wellbore. The casing is used as a conduit for well cementation and a pressurized container for production tubing.
A casing section is normally a seamless steel tube approximately forty feet in length, anywhere from four and one-half to twenty inches in diameter and may have a wall thickness in excess of one inch. The casing section is normally threaded at each end with a collar screwed on to one end in preparation for placing in a well bore.
Referring to FIG. 1, a wellbore 4 is of a significantly larger diameter than the outer diameter of a casing section 5 to allow easy placement down hole. A casing section 5 is placed in an upright position and lowered partially downhole, with a collared end 6 extending above the ground surface surrounding wellbore 4. Casing section 5 is held in place by slips 7 secured to the surface to prevent casing section 5 from further descending wellbore 4. A second casing section 5A is stood upright with its uncollared end approximately in line with collared end 6 of the previous, partially downhole section 5. Casing 5A is stood upright by a block arrangement 8 connected to a flexible cord 9 secured to casing 5A. A man referred to as the "stabber" (not shown) is located on a platform 10 on a drilling rig 11 thirty to forty feet above the ground. When the casing 5A is vertical, he throws a rope around collared end 6A of casing section 5A and attempts to line it up with collar 6 of casing section 5. Hydraulic tongs (not shown) are connected to casing section 5A and it is rotated along its center line to screw into the exposed collar 6 of the previous casing section 5. Casing section 5A is lowered into wellbore 4 and held in place by slips 7 which had been loosened to permit lowering of casing 5A and tightened to hold casing section 5A. The procedure is repeated until casing sections the length of the wellbore have been put in place.
Modern hydrocarbon wells are of increasing depth and a well twenty thousand feet deep is not uncommon. This depth requires five hundred casing sections or approximately five hundred joints, where one casing section is married to another.
Since the "stabber" is forty feet from the joint and must be significantly far from the centerline he is attempting to coincide, misalignment problems can often occur. Misalignment of one inch at the stabber position can damage threads to prevent a positive seal between casings. Misalignment of four inches at the stabber position will gall the threads and ruin the seal between casing joints. Misalignment of twelve inches at the stabber position will result in crossthreading.
In previous hydrocarbon production wells, depths of only a few thousand feet were common and a seal problem was of minimal concern since high pressure is not associated with shallow wells. Presently, casing sections must be able to withstand many thousand pounds of pressure and a poor seal may washout surrounding formations despite cementing the casings in place.
Furthermore, a casing section may weigh as much as ninety pounds per foot. While the casing may be rated to hold six hundred tons and the joint strength may be eighty percent or four hundred eighty tons, a joint made up having its threads damaged or galled will be significantly less. As a result, a joint may separate sending several thousand feet of casing downhole. The casing must either be recovered or a smaller diameter casing lowered within the casing that was dropped. Since a fall of more than ten thousand feet, (approximate two miles) may have damaged and broken the solid casing joints, several millions of dollars extra may be required to complete the well.